This invention relates to an apparatus for detecting a rotational position or a rotational speed of a rotary body such as a rotor of an electric motor.
Synchros called resolvers or selsyns, etc. are widely used for the detection of revolution of electric motors because they are similar in structure to electric motors and have high reliability. For constituting an apparatus for detecting revolution using a synchro, it is required to combine the synchro with a converter, e.g., a synchro/digital converter (S/D converter), or a resolver/digital converter (R/D converter), etc.
Two kinds of representative converter systems are known. One system is a system called the tracking type converter, for example, IS14, IS24, IS44 and IS64, etc. by ANALOG DEVICES Inc. When the primary winding of a synchro is subjected to single-phase excitation, an induced voltage signal having an amplitude determined by the sine-wave and cosine-wave functions corresponding to a rotational position is produced on the two-phase secondary winding. In the converter, a type of phase locked loop is constituted so that sine-wave and cosine-wave signals prepared by the count value of an up/down counter follow the amplitude of an induced voltage signal varying in a sinusoidal manner by changes in the rotational position. By conducting an operation such that the count value of the up/down counter follows the rotational position to read the count value therefrom, the rotational position can be obtained as a value converted to a corresponding digital value. This system is able to read rotational digital value. This sytem is able to read rotational position at all times. However, not is this system complicated in its construction and expensive, but also it includes many analog elements, thus making it difficult to realize an implementation as an integrated circuit.
The other system is a system called the phase detection type converter. When the two-phase primary winding of a synchro is excited by a two-phase sine-wave signal, a sinusoidal induced voltage signal which changes phase in accordance with the rotational position is produced on the secondary winding thereof. By detecting a phase change in the induced voltage signal with respect to the excitation sine-wave signal, the rotational position is detected.
This phase detection type converter is simple in circuit configuration and most parts thereof are configured as a digital circuit, thus being capable of being realized at a low cost. With this sytem, however, the value detected is only updated every zero point of a secondary induced voltage of the synchro, so that when the rotational position changes, the detection period fluctuates in dependence upon the rotational speed. Accordingly, since it is impossible to establish synchronization with the sampling period for digital control, it is also difficult to apply this system to the digital control of electric motors. In addition, in the case of obtaining a higher resolution of detecting rotational position of multi-revolution using a combination of synchros having different poles called multiple-speed synchros, or of a plurality of synchros decelerated by reduction gears, since respective detection timings are different from each other, it is difficult to synthesize a plurality of detected values when the rotational speed becomes high.